Counterflow in situ combustion process



Sept. 18, 1962 J. N. DEW l-:TAL

COUNTERFLOW IN SITU COMBUSTION PROCESS Filed April 17, 1958 INVENTOR.JOHN N. DEW WILL/AM L. MART/N Sept. 18, 1962 J. N. DEW ETAL COUNTERFLOWIN SITU COMBUSTION PROCESS Sept. 18, 1962 J. N. DEW ET AL 3,054,448

couNTERFLow 1N SITU coMBUsToN PROCESS Filed April 17, 1958 esheets-sheet 3 IOO VISCOSITY, CENTIPOISE TEMPERATURE F INV ENTOR.

JOHN N. DEW WILLIAM L. MART/N A TTORNE Y Sept. 18, 1962 J. N. DEW ETALcouNTERFLow IN SITU coMBUsTIoN PROCESS 6 Sheets-Shea?I 4 Filed April 17,1958 a o0 m fa/f/ mmum9a7e54s2 n o InFlol .:Q n o zocbo .SFME. fic

Sept. 18, 1962 J. N. DEW ET AL couNTERFLow 1N sITU coMBUsTIoN PROCESSFiled April 17, 1958 6 Sheets-Sheet 5 /V\ 0 GOV//f/mv m lOO NVENTOR.JOHN N. DEW WILL/AM L. MART/N im ATTORNE Sept. 18, 1962 Filed April 17,1958 RELATIVE INJECTION PRESSURE (Ph/Pc) J. N. DEW ETAL 3,054,448

COUNTERFLOW IN SITU COMBUSTION PROCESS 6 Sheets-Sheet 6 INVENTOR. JOHNNA DEW WILLIAM L. MART/N ATTORNEY United States Patent tltice 3,054,448Patented Sept. 18, 1962 3,054 448 COUNTERFLOW 1N SITU ,COMBUSTIONPROCESS John N. Dew and William L. Martin, Ponca City, Okla., assignorsto Continental Oil Company, Ponca City, Okla., a corporation of DelawareFiled Apr. 17, 1958, Ser. No. 729,156 4 Claims. (Cl. 166-11) Thisinvention is concerned with thermal recovery of crude oil fromstratigraphic formations and is more particularly directed to theproduction of oil by the use of counterflow in situ combustion in aseries of juxtaposed oil bearing formations separated by verticalpermeability barriers.

In situ combustion processes are generally known to the art and involvethe injection of an oxygen containing gas into a reservoir through -aninput well to develop an air or oxygen containing gas sweep through thereservoir and to a production well, raising the temperature of theformation lat the injection well to the ignition point by utilizing aheater at the injection face of the formation or by other suitablemeans, and propagating the resulting combustion front through thereservoir by continued gas injection. As the combustion front progressesoutwardly from the injection well, formation waters and most of the oilimmediately in advance of the combustion zone are vaporized and sweptout into the reservoir by the resulting combustion gases. A portion ofthe oil which remains in liquid phase is also urged through thereservoir by the imposed -gas drive. The oil which remains in front ofthe combustion front is 'a higher boiling fraction which, because of thehigh temperatures existing in this location, is altered to a coke-likematerial which remains in place and is utilized as fuel to propagate thecombustion front. In order to maintain combustion we have found itnecessary to inject oxygen at a suicient rate to maintain a temperaturein the combustion zone of at least 600 F.

It will be apparent that at this temperature, fractionation of the oilas well as vaporization of all interstitial water occurs. Since thetemperature decreases rapidly immediately in front of the combustionfront, fractional condensation of the oil and condensation of the waterwill occur a relatively short distance outwardly from the combustionfront. There -is thus a substantial increase in liquid saturations in azone where the temperature and oil mobility are low. Consequently,excessive liquid saturation builds up and increased resistance to gasilow quickly develops. There is, in eect, a saturation by liquid of thechannels through which gas ilow has been maintained. The occurrence ofthese zones of high liquid saturation and low gas lflow is generallyreferred to as the formation of permeability blocks. The tendenoy ofpermeability blocks to develop during in situ combustion results in thenecessity for increasing injection pressures in order to maintainsufficient ilow rates through the combustion zone to permit combustion.Intermittent occurrence of permeability blocks thus constitutes acondition detracting markedly from the economy of oil production by insitu combustion methods.

It is further significant that although oil production rates may beincreased by in situ combustion, any increases in production rates ofrecovery wells are almost entirely the result of the pressure effects ofthe imposed gas drive. Production rates are, nevertheless, verysubstantially depen-dent upon the permeability of the formationsurrounding the production well and the viscosity `of the oil in itsvicinity. Furthermore, as is usually the case, the air or oxygencontaining gas introduced into the injection well does not all leavelthe formation through the production well but may escape into otherareas of the formation thus sweeping a portion of the oil which it isdesired to produce into areas from which it cannot readily be recovered.As a result of these additional factors, conventional in situ combustionprocesses require substantially greater quantities of air per barrel ofoil recovered than would be necessary in their absence.

It is therefore an object of our invention to substantially increase theeconomy of oil recovery by decreasing the quantities of air necessary toproduce a formation by in situ combustion.

It is another object of our invention to decrease the viscosity of oilin the area around the production well, thus substantially increasingproduction rates and decreasing the cost of oil recovery.

We further desire to provide a method which will substantially decreasethe time lapse between initiation of in situ combustion and subsequentrecovery.

We also wish to furnish a method which increases the effectivepermeabilities of the producing zone to gas flow and thus diminishes thecompressor horsepower required for -air injection.

It is a further object of our invention to permit a decrease in the wellpattern density without causing a decrease in production rates bydecreasing the viscosity of the crude oil in the vicinity `of theproduction well, by increasing the gas viscosity, oil volume, and gasvolume in the producing strata near the production Well, by eliminatingpermeability blocks and by causing directionalization of ow patternsbetween injection and producing walls.

These, Ias well as further objects, will be apparent from aconsideration of the `following description as related to the drawing inwhich:

FIGURE 1 is a block diagram of the preferred arrangement of Wells forpracticing our method;

FIGURE 2, is a diagrammatic elevational view of an alternate arrangementof Wells which can be utilized in practicing our process;

FIGURE 3 is a graph showing the viscosity in centipoises, at differenttemperatures, of a 21 API crude oil recovered from the North TisdaleField in Johnson County, Wyoming;

FIGURE 4 is a graph illustrating the eiects of the temperature andradius of a heated zone surrounding a production well on the relativeproduction capacity of that well with respect lto another well having noheated zone;

FIGURE 5 is a graph illustrating the eifects of the temperature andradius of a heated zone surrounding a production well on the relativespacing between injection and production wells required to give aconstant llow rate for an in situ combustion system employing thecounterflow principles of our invention and for a conventional in situcombustion system;

FIGURE 6 is a graph showing the elects of the temperature and radius ofaheated zone surrounding a production well on the relative injectionpressures required to `give a constant flow rate for Ian in situcombustion system employing the principles -of our invention and for aconventional in situ combustion system;

FIGURE 7 is a schematic representation of oil ilow patterns between theinjection well and production wel] of `a conventional in situ combustionsystem; and

FIGURE 8 is a schematic representation of oil ow patterns between `aninjection well and production Well of our counterflow in situ combustionprocess.

Referring more specifically to the drawing and especially to FIGURE l, aplurality of wells 1, 2 and 3 traverse oil bearing sands or productionzones 4 and 5, which are separated by impermeable strata 6. The wellsalso, of course, penetrate an overburden 7 as well as anotherimpermeable strata 8 immediately above produc. tion zone 4. The wells l,2 and 3 may terminate within production zone 5 or, in the event thatthey penetrate the underburden 9 or other formations which may or maynot contain oil below the production zone 5, the wells may `be sealedfrom communication with such strata by utilization of packers as is wellknown in the art. well is illustrated as being provided with an outercasing 10 which is cemented to the bore of the drilled hole in thecustomary manner. The casing 10 extends downwardly through thejuxtaposed oil bearing sands 4 and 5 and is perforated as at 11 topermit uid ow therethrough. A tubing string l12 extends downwardly intothe well bore within the casing and is open at its lower end. The tubingstring 12 is provided with an inlet 14 for compressed air or oxygencontaining gas which is generated by suitable equipment (not shown) onthe surface. Insofar as wells 2 and 3 are concerned, each has an airinlet 16 which is connected to the casing 10 while the tubing string 12in each of these wells is provided with an outlet 18 for the producedoil. Packers are positioned between the tubing string and the casing 10in the vicinity of the impermeable strata 6 in order to preventcommunication between oil sands 4 and 5.

In accordance with our method, compressed air or gas is introducedthrough the tubing string 12 of well 1 and admitted to the lowermost oilbearing strata, oil sand 5. Communication between well 1 and wells 2 and3 through oil sand 5 is established by maintaining gas pressure until aow of gas is obtained through the tubing strings 12 of wells 2 and 3.Heat is then applied at the formation face in the form of an electric orgas heater or through combustion of a material such as charcoal in orderto raise the temperature of the incoming air or oxygen containing gas tothat required to initiate combustion of the hydrocarbon materials inplace at the face of formation in adjacency with well bore 1.

Substantially concurrently with the initiation of combustion in`oil sand5, combustion is initiated in the oil sand I4 in adjacency with wells 2and 3 by proceeding in the same manner as set forth above with respectto well 1.

As soon as combustion is started in the formations 4 and 5, heattransfer from the area of combustion in one zone to the immediatelyjuxtaposed area in the adjacent formation begins. Because of air flow ina direction away from the well and because of the rapid condensation ofliquids immediately in advance of eachrof the combustion zones, themajor portion of the heat resulting from combustion is transferred byconduction to the earth strata immediately above and immediately belowthe combustion zone. The heat which is so transferred decreases theviscosity of the oil, first in an area immediately around the Well boreand successively outwardly as combustion progresses. This increases theeffective radiusl of drainage of the production tubing of the well andrapidly increases rates of production without increasing airrequirements. Still referring to FIGURE l, combustion has progressedradially on oil sand 5 outwardly Ifrom well 1 and in a direction towardswells 2 and 3. Similarly, in oil sand 4, combustion has progressedoutwardly and radially yfrom wells 2 and 3 towards well 1. Therepresentation of conditions within each of these zones as a result ofthe in situ combustion is necessarily schematic but is neverthelesssuiciently representative for functional discussion. As combustionprogresses outwardly from the well a radially extending combusted zone`21 will be found in an area about each well. This zone has beeneffectively depleted of all of its oil and water, nothing remaining butporous media.

Spaced outwardly annularly from, and adjacent to the combusted zone, isa combustion zone 22 in whichl combustion of residual oil fractionsoccurs to produce suiicient heat to vaporize most of the oil occurringintheV sand 5 as well as to vaporizeV all interstitial water. 'Ihevaporized oil and'water vapor condense at the'trailing edge of an areaor zone, referred to on the drawing by the designation liquid bank 24,in front of the combustion zone. As previously indicated, although thereis heat transfer EachV Y bustion therein.

from the combustion zone through the liquid bank and into the remainingunaiected portion 26 of the oil sand and hence some decrease inviscosity of oil in the latter area with a concomitant increase inproduction rate, the majority of the heat of combustion is transferredvertically into immediately adjacent formations through the impermeablestrata 6 and into the oil sand 4 in the area of the oil sand which hasnot been laterally affected by com- As combustion progresses radiallyfrom well 1, the temperature of the oil in oil sand 4 increasesprogressively in a radial direction so as to coincide substantially withthe temperature rise in formation S thus decreasing the viscosity ofliquids in `oil sand `4 and increasing the mobility of the oil throughthe formation. What occurs in formation 4 as a result of combustion information 5 also occurs in formation 5 as a result of combustioninformation 4 in the area of wells 2 and 3. As to formation 5, well 1 isan injection well and wells 2 and 3 are production wells. Contrariwise,with respect to formation 4, wells 2 and 3 constitute injection wellsand well 1 constitutes a production well.

Although it is preferred to utilize a single well bore to accomplishboth injection and production with respect to dilerent adjacentformations, FIGURE 2 illustrates a suitable arrangement for practicingthe method of our invention by employment of separate Well bores for airinjection and for oil recovery. IFor example, in FIGURE 2 the wells 30,32, 34 and 36 penetrate a plurality of stratigraphic formationsincluding theV overburden 38, oil sands 40, 42, 44, 46 and '48 andimpermeable strata 50, 52, `54, 56, 5S and 60 which form verticalpermeability barriers between otherwise adjacent formations. Wells 30and 36 constitute air injection wells as to alternatively adjacentformations, the well 30 having a casing `62 which is perforated as at'64 formations `4I), 44 and 48 while the injection well 36 has a casing66 which is provided with perforations `68 within formations 42 and 46.The oil recovery wells 32 and 34 are each respectively adjacent toinjection wells 30 and 36 and the casings of these wells `69 and 70 are-similarly perforated in alternate formations. The casing y69 isperforated as at 72 within lformations `42 and 46 while the casing 70 ofrecovery well 34 is perforated as at 74 within formations 40, 44 and 48.

As indicated with respect to the operation of the well arrangement ofFIGURE l, in situ combustion is initiated in formations 40, 44 and 48 inlche area adjacent to lthe injection well 30 by injecting air underpressure through this well to obtain an air `sweep through each of theformations to the recovery well 34. The air is heated to the combustiontemperature of the hydrocarbons occurring in these formations and ltheresulting combustion front is propagated through such formations bycontinuance of air injection. As combustion progresses, each of theseformations is provided with a radially expanding combusted zone 76, yacombustion zone 78, and a liquid bank 80. Similar zones referred to bythe same reference numerals progress radially ouftwardly from theinjection Well 36. Since the `air injection wells 30 and 36 arerespectively adjacent to oil recovery wells 32 and 34, the 'oil in eachproducing section of the hydrocarbon bearing formations will be heatedas a result of vertical conductance of heat from the combustion zone 78of the immediately adjacent formation or form-ations. Where there are -alarge plurality of juxtaposed formations Separated by verticalpermeability barriers, ysuch as is the case 1n the San Miguelito Field nCalifornia, the alternate production zones adjacent to the recoverywells 32 and 34 will be vertically heated by conductance from twoadjacent combustion zones.

It be clear from a consideration of FIGURES l and 2 that multiple pairsor combinations of adjacent oil sands can be produced by utilization ofconventional multiple completion wells in which each of a plurality oftubing strings communicate with a single oil sand. In such a case, Iasingle well may constitute an injection well as to one or more producingzones while constituting a production well as to one or more otherproducing zones.

The substantially improved effectiveness of ourcountercurrent in situcombustion method is demonstrated in FIGURES 3 through 6 for a 21 APIcrude oil recovered from the North Tisdale Field in Johnson County,Wyoming.

FIGURE 3 is, as noted, a plot of the viscosity in centipoises (asordinate) of the 21 API North Tisdale crude as a function of temperature(as abscissa). From this curve, 82, it will be seen that the viscosityof the oil at the production zone temperature of 70 F. is approximately300 centipoises; that at 100 F. its viscosity is approximately 125 op.While at a temperature of 150 F. viscosity is approximately 40 cp. andat a temperature of 300 F. viscosity is approximately 5.5 cp.

The graphs of FIGURES 4, 5 and 6 have been developed for a 21 API crudeoil, a well bore radius of 4" and yan effective radius ofi drainage otan output Well equal to the distance between the input and output wellsin a 5-spot developed pattern divided by the factor 1.855, byutilization of the following formulas. The development of theseequations from standard formulas described in chapters 4 and 9 inMuskat, The Flow of Homogeneous Fluids Through Porous Media (l. W.Edwards, Inc., 1946), will be clear to those familiar with the art.

For FIGURE 4- u2 ln re/rw Qc-ul ln r/rwJl-uz ln re/r For FIGURE 5- (re)hL) (re)c rw For FIGURE 6 where:

It will be seen from FIGURE 4 that the relative production capacities oftwo independent wells, each one occupying the `same position in aso-called S-Spot developed pattern, and one of which is heated for apercentage of the dist-ance outwardly to an injection well while theother of which is maintained at its original reservoir temperature,increases substantially for any single temperature as the radius of theheated zone increases with respect to the distance between theproduction and injection wells. For example, it will be seen from curve84 that ata temperature of 100 F., the production rate of a well whichhas a heated zone radius .1 the total distance between the productionand the injection Wells is approximately 1.8 times the production rateof a well having a relatively constant temperature of 70 F. between theproduction and the injection Wells. Similarly, it will be seen fromcurve S6 that if this same Zone is heated to a temperature of 150 F.,the relative production rate or capacity will be 2.9 while curve S8shows that at a temperature of 300 F. the relative production rate willbe 3.9. It Will also be noted that for any one of these temperatures therelative production capacity of the heated well with respect to theunheated well increases as a function of the ratio of the radius of theheated zone to the distance between the injection and the productionwells. For example, for a temperature of the heated zone of 300 F. theflow rate of the heated zone well is slightly less than yfour times theproduction rate of a well not having a heated zone where the radius ofthe heated zone is .1 of the distance between the production and theinjection wells, while where this latter radius is .4, the ow rate ofthe Well having a heated zone is approximately 15 times that of a wellnot having a heated zone.

FIGURE 5 is a graph illustrating the eiect of the heated zone radius andthe temperature of that Zone on the relative spacing required forequivalent flow rates from producing wells. Referring speciiically tocurve 90, heating a zone having a radius of 20 feet to a temperature ofF. results in a permissible spacing ten times that possible with aproduction well having no heated zone. The temperature and heated zoneradius are even more noticeable with respect to the curves 92 and 94.Simply heating a zone 10 feet in radius to a temperature of 150 F.results in a permissive spacing between injection and production wellsof 18 times that required to get the same production from a Well nothaving -a heated zone, While if this saine 10-foot radius zone is heatedto a temperature of 300 F., the spacing of injection and productionwells can be 28 times that required to maintain the `saine ow ratesbetween an injection and a production Well not having a heated zone. Ofcourse it will be immediately recognized that substantial economies inproduction `costs can 'be obtained because of the ability to increasethe spacing between production and injection wells Without decreasingproduction rates.

The graph of FIGURE 6 shows the effects of the heated zone radius andits temperature on the relative injection pressure required forequivalent ilow rates from production wells having a heated zone and nothaving a heated zone. It will be seen that each of curves 96, 98 and10i? indicate a rapid decrease in the required relative injectionpressure as the ratio of the radius of the heated zone to the distancebetween the injection and production wells increases. Thus, in order tomaintain the same production rates for two Wells, a rst of which has aheated zone and the other of which does not, it is only necessary tomaintain an air injection pressure which is .6 that of the injectionpressure required for the Well not having a heated zone where the firstwell has a heated zone having a temperature of 100 F. and a radius whichis .1 the distance between the injection and production Wells. Theeffect of temperature and the radius of the heated zone is even moreapparent with reference to curve 100. As shown by this curve, the sameilow rates can be obtained from two different production wells subjectedto otherwise identical conditions notwithstanding the fact that one Wellsystem requires .l the air injection pressure required by the other Wellsystem, if the temperature of a zone having a radius equal to .3 thedistance between injection and production wells is raised to atemperature of 300 F.

The foregoing graphs convincingly demonstrate the substantial advantagesaccruing as a result of utilization of our counterow in situ combustionprocess. Our process may be employed either to increase productioncapacities while maintaining conventional spacing between injection andproduction wells and maintaining conventional injection pressures or toincrease spacing or to reduce required air injection pressures whileobtaining substantially the same production.y It is also possible tovary two or even three of the foregoing factors to obtain greatlyincreased economy of operation. Increasing production rates decreasesthe total time required to produce a particular eld, while increasingthe relative spacing of injection and production wells substantiallydecreases the capital outlay required for wells to produce a part-icu-17 lar eld, and decreasing air injection pressures reduces operatingcosts and capital outlays-for compressors and related equipment.

FIGURES 7 and 8 are further illustrative of the mechanism of ourprocess. In FIGURE 7, where` conventional in situ combustion ow patternsare schematically illustrated, crude oil flows from injection well 102to production wells 104 and 106. It will be evident that the lines ofoil flow 108 from injection well 102 to the production wells aresubstantially straight along the shortest paths between such wells. Inan area spaced from this direct path there is some directionalization ofoil flow as indicated by the .flow lines 110. However, in the areamidway between the production wells, flow lines 112 from the injectionwell 102 again tend to become radial straight lines. In short, FIGURE 7illustrates that with conventional gas injection methods, a signiicantquantity of the injected gas is wasted in dispersing the crude oilthroughout the formation rather than in directing it towards theproducing wells. On the other hand, FIGURE 8 illustrates thedirectionalization effects of our counterow in situ combustion method.Here, considering that this ligure represents conditions of flow inproduction zone of FIGURE l, the well 1 is, as to this particularformation, an injection well while the wells 2. and 3 are, as to thisformation, production Wells. The mechanism of the directioualizationeffects of counteroW in situ combustion can be schematically illustratedby the dashed line circles 114 and 1116 around production wells 2 and 3.The increased temperatures inthe producing zones around these wells,resulting from heat conductance from combustion in the adjacent strata,results in substantially reduced viscosity of the oil in situ andincreased mobility of the strata to the iiow of oil and gas. The eiectsof decreased oil viscosity and substantially increased gas and oilmobility are substantially the same asV if the diameter of the wellbores of Wells 2 and 3 were substantially increased. Thus the dashedcircles 114 and 1116 may be considered to represent the effective wellbore of Wells 2 and 3 lresulting from our counterow in situ combustion.It wil1 -be clear that the direction of flow of oil at a point 118 willbe as indicated by the oil ilow line 120 which results in production ofoil from this point whereas the direction of flow of oil occurring at apoint 122 (in FIGURE 7) will be as indicated by line 124 and Will not beproduced by the corresponding product-ion well 104.

Having fully described the details of our process andV its advantages,We claim: j

1. A method of producing hydrocarbons from two vertically-spacedadjacent hydrocarbon-bearing zones within a reservoir separated by asubstant-ially impermeable barrier which comprises drillingtwospaced-apart Vwell bores into said reservoir whereby said zones aretraversed, injecting combustion-supporting gas through a first of saidwell bores into the irst of said zones, injecting combustion-supportinggas through the second of -said well bores into the second of saidzones, initiating in situ combustion in said rst zone in the vicinity ofsaid first Well bore and in said second zone in the vicinity of saidsecond well bore, maintaining the iiow of said combustion-supporting gasthereby propagating combustion in each of said zones counterow to thedirection of combustion in the adjacent zone, producing the hydrocarbonsof said second zone through said iirst well bore, and producing thehydrocarbons of said firstA zone through said second Well l`bore.

2. A method of producing hydrocarbons from two ver.-

E tically-spaced adjacent hydrocarbon-bearing zones within a reservoirseparated by a substantially impermeable barrier which comprisesdrilling three spaced-apart well boresV into said reservoir whereby said'zones are traversed, injecting combustion-supporting gas through aiirst of said well bores into the rst of said zones, injectingcombustion-supporting gas through the second and third of said wellbores into the second of said zones, initiating in situ combustion insaid rst zone in the vicinity of said first Well bore and in said secondzone in the vicinity of said second and third well bores, maintainingthe ow of said combustion-supporting gas thereby propagating cornbustionin each of said zones counterow to the direction of combustion in theadjacent zone, producing the hydrocarbons of said second zone from saidfirst well bore and producingl the hydrocarbons of said first zone fromsaid second and third Well bores.

.3. A method of producing hydrocarbons from three, an upper,intermediate and lower zone, lvertically-spaced adjacenthydrocarbon-bearing zones within a reservoir separated by substantiallyimpermeable barriers which comprises drilling two yspaced-apart wellbores into said reservoir whereby said zones are traversed, injectingcombustion-supporting gas through a first of said well bores into theintermediate of said zones, injecting combustionsupporting gas throughthe second of said well bores into the upper and lower of said zones,initiating in situ combustion in said intermediate zone in the vicinityof said iirst well bore and in said upperand lower zones in the vicinityof said second well bore, maintaining the ilow of saidcombustion-supporting gas thereby propagating combustion in each of saidzones counterflow to the l direction of combustion in the adjacentzones, producing the hydrocarbons of the upper and lower zones throughsaid rst well bore, and producing the hydrocarbons of the intermediatezone through said second well bore.

4. A method of producing hydrocarbons from three, an upper, intermediateand lower zone, vertically-spaced adjacent hydrocarbon-bearing zoneswithin a reservoir separated by substantially impermeable barriers whichcomprises drilling three spaced-apart well Vbores into said reservoirwhereby said zones are traversed, injecting combustion-supporting gasthrough a first of said well bores into the intermediate of said zones,injecting combustionsupporting gas through the second and third of saidwell bores into the upper and lower of said zones, initiating in situcombustion in said intermediate zone in the vicinity of said iirst wellbore Vand in said upper and lower zones in the vicinity of said secondand third Well bores, maintaining the How of said combustion-supportinggas thereby propagating combustion in each of said zones counterflow tothe direction of combustion in the adjacent zones, producing thehydrocarbons of said intermediate zone from said second and third wellbores, and producing the hydrocarbons of said upper and lower zones fromsaid irst well bore.

References Cited in the le of this patent UNITED STATES PATENTS2,217,749 Hewitt Oct. 15, 1940 2,238,701 McCollum Apr. 15, 19412,561,639 Squires July 24, 1951 2,584,605 Merriam et al. Feb. 5, 1952'2,642,943 Smith June 23, 1953 2,734,579 Elkins Feb. 14, 1956 2,736,381Allen Feb. 28, 1956 2,793,696 Morse May 28, 1957 :UNITED STATES PATENTOFFICE CERTIFICATE GF CORECTION Patent No 3,054,448 September 18, 1962John N Dew et aL,

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 2, line 28, for "walls" read wells column 3, line 55, for "on"read .in

Signed and sealed this 2nd dey of April 1963a (SEAL) Attest:

ESTON o, JOHNSON l DAVID L. LADD Attesting Officer Commissioner ofPatents

